The present invention generally relates to packaging integrated circuits, and more particularly to joining a chip to a substrate with two or more different solder alloys.
A semiconductor chip may be joined to a substrate (also known as a composite substrate, a laminate substrate, or an organic laminate substrate) using a plurality of solder connections to form a semiconductor package.
An example of the solder connection includes controlled collapse chip connection (also known as C4 or flip-chip connection). Generally, solder connections may generally include an array of small solder balls on the surface of the chip before the chip is joined to the substrate. More specifically, each individual solder connection may include a bonding pad on the chip, a solder bump, and a corresponding bonding pad on the substrate. A typical joining sequence may begin with depositing or applying a plurality of solder bumps on a plurality of bonding pads on the chip. The plurality of solder bumps are then heated to a temperature sufficient to cause them to reflow. Next, the chip, including the plurality of solder bumps, is aligned to and placed on a chip site on the substrate. In doing so, the plurality of solder bumps contact the plurality of bonding pads on the substrate. The plurality of solder bumps are again heated to a temperature sufficient to cause them to reflow. The final solder connections may electrically connect and physically join the chip to the substrate and create a semiconductor package.
The primary function of the solder connection is to electrically connect and physically join a chip to a substrate. Further, each individual solder connection may serve a different electrical function, or may form a different type of solder connection. For example, some solder connections may be used for grounding or to supply higher current voltages and subsequently form a voltage connection between the chip and the substrate, while other solder connections may be used to transmit signal data and subsequently form a signal connection between the chip and the substrate. In general, the signal connections may experience relatively low current density under normal operating conditions and the voltage connections may experience relatively high current density under normal operating conditions. Power transfer and signal transmission may be examples of specific operational requirements of a particular solder connection.
Typical solder connections use a lead-tin (Pb/Sn) solder with high lead content of 95% or more, by weight. The tin content is kept to a minimum, usually less than 5%, by weight, because tin may undesirably react with the copper and form undesirable intermetallics. The higher the tin content, the more the solder reacts with the copper. Consequently, there may also be a nickel (Ni) or cobalt (Co) barrier layer to either protect the copper layer from being consumed by the solder or to protect various layers underneath the copper layer.
Solder alloys containing lead are now recognized as being harmful to the environment, and there is a considerable interest in using lead-free solder alloys. For example, two commercially-available lead-free solder alloys include tin-silver (Sn/Ag) and tin-silver-copper (Sn/Ag/Cu). Current technologies rely on a single solder alloy to form all the solder connections used to join a chip to a substrate. For example, all the solder connections between the chip and the substrate may include a tin-silver-copper solder alloy having a silver content of 1.4%, by weight, and a copper concentration of 0.5%, by weight.